The present invention relates to optical elements, and, more specifically, to an optical waveguide having a plurality of periodic sub-wavelength sized regions or gratings in which the light propagates substantially parallel to the regions or gratings.
Guiding elements for electromagnetic radiation on planar substrates have typically been provided by one of two methods. One is by confining the light within a material having a relatively high dielectric constant (“core”), which is surrounded by a material having a lower dielectric constant (“cladding”). Another is by photonic crystals in which a two-dimensional periodic arrangement of materials with high and low dielectric constants creates a photonic bandgap that surrounds a defect line (“waveguide”). Confinement normal to the substrate is either provided by a transparent layer with a dielectric constant between the dielectric constant of the photonic crystal and the dielectric constant of the substrate/cladding or guiding (equals the total internal reflection) in the material of the photonic crystal itself.
A problem with the light confining approach is that the core material must have a relatively low absorption in the wavelength range to be transmitted. Furthermore, a relatively high contrast of the dielectric constants between the core and the cladding is required for achieving a relatively high areal integration density of the waveguides. Hence, crystalline semiconductors such as Si, GaAs or InP are typical materials for the core. However, they are not suitable for the visible wavelength range because of their absorption characteristics.
A problem with the photonic crystals approach is that the light penetrates into the photonic crystal structure (e.g., a two-dimensional periodic hexagonal array of holes) and decays exponentially. Hence, the material for the photonic crystal must have relatively low absorption in the wavelength range to be transmitted. Because of the required relatively high contrast of the dielectric constants between the materials in the photonic crystal (i.e., holes vs. bulk), crystalline semiconductors such as Si, GaAs or InP are typically used. However, these semiconductor materials are in general not suitable for the visible wavelength range because of their absorption characteristics. Further, the photonic crystal structure extends considerably transverse to the guiding direction (4-5 lattice periods or more), which prevents the realization of relatively dense areal integration.